Active sonars emit signals which, in reality, have a limited duration T and occupy a frequency band B which is likewise limited. These band and duration characteristics, can be exploited to obtain an amplitude processing gain, dependent on the product B*T, and an ability to separate the signals received in the time domain, with a resolution dependent on 1/B and in the frequency domain with a resolution dependent on 1/T. More precisely for each sonar it is possible to define an ambiguity function which represents its ability to distinguish echoes originating from closer or more distant sources (the distance being manifested by a propagation delay) and going more or less quickly (the speed being manifested on account of the Doppler effect by modification of the frequency spectrum).
Broadband signals exhibit the benefit of allowing detection, distance estimation and distance resolving power that are all the better the broader the band. Among these broadband signals, signals modulated hyperbolically in frequency have the advantage of being insensitive to Doppler impairment: stated otherwise even affected by the Doppler effect, the reflected signal is in the image of the signal emitted and a target will be detected by the same detector regardless of its speed, the processing gain being unchanged. However, this insensitivity is accompanied by a “Doppler-delay” uncertainty which has the consequence that, without information about the distance, one does not know precisely how to estimate the speed and conversely.
The line spectrum signals have on the contrary the advantage of separating echoes well as a function of the relative speed of the sonar and of the targets, the Doppler effect being manifested by a shift or “slip” of the frequency of the signal received with respect to the frequency emitted. This property is especially utilized to combat reverberation noise. The reverberation results from the reflection of the signal on multiple heterogeneities in suspension or on the bed or else on the surface. These reflectors being fixed the corresponding signals received in a channel of the sonar are affected by a Doppler which depends only on the speed of the carrier and the direction pointed at by the reception channel of the sonar. For a mobile target the same holds but the Doppler effect is increased on account of the target's own speed with respect to the carrier. By simple filtering it is thus possible to differentiate an echo from the reverberation and to estimate the relative speed of the target that returned this echo provided that the frequency resolution of the sonar is sufficient.
Contemporary sonars generally operate according to one or other of these modes by emitting either broadband signals, or line spectrum signals. Thus application FR 03 04042 filed on Jan. 4, 2003 by the Applicant, describes a sonar which simultaneously uses the properties of frequency modulated signals and of signals with high Doppler resolution such as BPSK signals.
Sonars with line spectrum emissions make it possible to easily apply a Doppler processing to the signals received and to perform the classification of the echoes received, not only through the intensity of the corresponding signals received, but also through their Doppler frequencies. With respect to the carrier of the sonar, an echo is then characterized by three parameters, its distance with respect to the carrier, the bearing in which it is situated and its speed of displacement. The bearing may be defined as the angle made by the direction joining the sonar to the object whose echo is received with the heading of the carrier ship.
The speed parameter is in particular important for determining whether the object detected is liable to represent any threat for the carrier of the sonar. It also advantageously allows an improvement in the contrast. Specifically the echo reflected by a mobile target of small size situated in a zone of strong panoramic reverberation of the seabed, is invisible with a broad spectrum emission since its level is too low with respect to the level attained by the reverberation signal which uniformly covers the entire band. On the other hand, it may easily be separated from the ambient clutter constituted by the panoramic reverberation, by virtue of the frequency displacement due to the Doppler which results from the speed of the target. It may thus be detected and isolated.
However, the use of the Doppler parameter presents the operator with a problem of viewing the information received. Specifically in the absence of Doppler analysis, each echo may be portrayed on a plane, as a point, or a small surface, having two coordinates: its bearing and its distance. It is therefore possible, on the basis of a conventional viewing screen, to represent the echoes received. This representation may for example take the form of luminous spots whose position and size reflect the position and the size of the objects that have returned an echo. The level of the echo received being for example rendered by the intensity of the luminous spot, it is simple to make provision to acquire the parameters of a target by simple pointing of the corresponding echo.
On the other hand if a Doppler characterization of the echoes is carried out, each echo is then identified by three coordinates: its bearing, its distance and its Doppler frequency. A simple representation in a plane is then no longer possible.
A first solution then consists in representing the echoes detected in a three dimensional space. For this purpose it is for example possible to use a perspective representation, carried out along three axes, a distance axis, a bearing axis and a Doppler axis. Such a representation is illustrated by FIG. 2. However, although exhibiting the advantage of allowing simultaneous representation of all the echoes received, the perspective representation exhibits serious drawbacks. Firstly it requires the use of special screens that are more complex and more expensive than straightforward conventional viewing screens. Next, such a representation renders the operations performed by the operators in charge of the sonar, such as the designation, by means of a cursor for example, of an echo displayed on the screen, more complex. Finally, in the case where a large number of echoes are received, the perspective representation loses its legibility.
A second solution, known from the prior art and commonly used, consists in simultaneously utilizing two simultaneous plane images. A first image, such as those of FIGS. 3 and 4, carries out the displaying of the echoes received having one and the same Doppler frequency, in a conventional bearing-distance plane. The desired Doppler frequency is selected by the operator. Such an image makes it possible to simultaneously view all the echoes having one and the same Doppler frequency. On the other hand the echoes exhibiting another Doppler frequency are not represented. The visual analysis of the echoes is therefore done by successive explorations of the various images available.
The representation in the bearing-distance plane is supplemented with a second image, such as that of FIG. 5, in a bearing-Doppler plane. This second image makes it possible to view the whole set of echoes situated at a given distance, regardless of their Doppler frequency in the Doppler span covered by the sonar. This distance is moreover selected by the operator. This second solution which consists of a pagewise visualization, has the advantage of being compatible with commonly used viewing screens. On the other hand on account of the fact that it requires the successive exploration of a large number of images in two different planes, it is lengthy and irksome for the operator to implement, even if the switch from the representation in one plane to the representation in the other plane is generally made easier by the implementation of computer tools. It is the cause of a certain number of interpretation errors especially as regards the detection of an object of relatively small size moving in zones of strong reverberation. Moreover such a representation does not allow global visualization of the panoramic reverberation of seabeds, the echoes manifesting this reverberation being distributed over all the images, as a function of their Doppler frequency. To be able to analyze each image properly, the operator must then perform a mental recomposition of the panoramic reverberation.